Nylon Based Matrix Systems
Nylon and Polypropylene Binary Blends
Several studies have investigated the binary blends of nylon with polypropylene (PP), including the use of compatibilizers, namely maleic anhydride grafted PP (MAPP) and acrylic copolymers (Agrawal, Oliveira, Araújo, & Melo, 2007; Huber, Misra, & Mohanty, 2014; Sathe, Devi, Rao, & Rao, 1996; La Mantia, 1993; Shashidhara, et al., 2009). Analysis of uncompatibilized blends have shown that nylon and PP are immiscible, which results in inferior mechanical properties that are below those expected based on the rule of mixtures (La Mantia, 1993; Shashidhara, et al., 2009). The miscibility of nylon and PP can be improved by adding a MAPP compatibilizing agent in amounts from 2-5 parts per hundred to binary nylon-PP blends.
Compatibilization with MAPP is also known to increase the ductility and impact strength of nylon/PP blends to above that of either neat polymer. However, even with the addition of MAPP, the tensile strength and Young's modulus of nylon/PP blends is reduced well below the strength and stiffness of neat nylon, and below the expected rule of mixtures (Shashidhara, et al., 2009). Shashidhara et al. showed that while the addition of MAPP improved the tensile performance of blends, they still underperformed compared to the theoretical properties determined by the rule of mixtures (Shashidhara, et al., 2009).
Based on these studies, nylon/PP binary blends are incompatible without the use of a compatibilizer. It can be expected that compatibilized nylon/PP binary blends can be achieved with improved impact strength, but at the cost of reduced tensile strength and stiffness compared to neat nylon and theoretical values.
Nylon/PLA Binary Blends
Two studies have been identified which investigated the binary blending of PLA and nylon, including the use of epoxy resin and maleic anhydride grafted polyethylene-octene elastomer as compatibilizing agents (Pai, Lai, & Chu, 2013; Wang, Hu, Li, Ji, & Li, 2010). Pai et al. investigated the reactive extrusion of 50/50 PLA/nylon 6,10 blends to improve compatibility between the polymers (Pai, Lai, & Chu, 2013). While the addition of epoxy resin resulted in a slight increase in properties compared to the neat binary blend, the tensile strength, Young's modulus, flexural strength, flexural modulus, elongation and notched impact strength were below the properties of either neat polymer. The only improved property was the un-notched impact strength. Wang et al. similarly found that while the poor miscibility of nylon and PLA could be improved with the addition of maleic anhydride grafted polyethylene-octene elastomer, the tensile strength and Young's modulus of the blend was inferior to either neat polymer (Wang, Hu, Li, Ji, & Li, 2010). In fact, the addition of the compatibilizer decreased the tensile strength and Young's modulus compared to the uncompatibilized binary blend—only the impact strength was increased to the equivalent of neat nylon.
Based on these studies, nylon/PLA binary blends are incompatible without the use of compatibilizer. The miscibility can be improved using compatibilizing agents, but the prepared blends can be expected to have inferior mechanical properties compared to either neat polymer, and below the expected theoretical values. The impact strength can be improved to the point that it is at most equal to that of neat nylon, however this comes at the cost of a drastic decrease in tensile strength and stiffness.
PLA/PP Binary Blends
Two studies have been identified which investigated the binary blends of PLA and PP (Hamad, Kaseem, & Deri, 2011; Yoo, et al., 2010). Hamad et al. investigated PLA/PP blends at 100/0, 70/30, 50/50, 30/70 and 0/100 ratios, and found the two polymers to be immiscible based on the interfacial tension of the blends, and tensile strength that was far inferior to the theoretical value based on the rule of mixing (Hamad, Kaseem, & Deri, 2011). They found that due to the poor miscibility, the binary blends had very poor tensile strength, but the Young's modulus was increased with the addition of the stiffer PLA phase. This study did not investigate any compatibilization of the blend. Yoo et al. studied similar binary PP/PLA (80/20 ratio) blends, but with the addition of MAPP and styrene-ethylene-butylene-styrene-g-maleic anhydride (SEBS-g-MA) compatibilizers (Yoo, et al., 2010). They found that the addition of MAPP improved the tensile strength of the blends slightly, but did not meet the expected value based on the rule of mixtures. However, MAPP addition had no effect on the impact strength of the blend. SEBS-g-MA had an opposite effect to MAPP—its addition to the blend decreased the tensile strength of the blend, but improved the impact strength.
Based on these studies, PP/PLA binary blends are incompatible, but miscibility can be improved with the use of compatibilizers, namely MAPP and SEBS-g-MA. However, even with the use of compatibilizers, there is a trade-off between increasing the tensile strength and the impact strength of the blend. Even with the use of the correct compatibilizing agent, the tensile strength is inferior to the value predicted by the rule of mixtures.
Based on the studies shown above, it should be expected that a ternary blend of nylon, PP and PLA will have poor performance, due to the inherent immiscibility/incompatibility of each polymer with each other. While compatibilizing agents have been shown in other studies to improve several properties of the binary blends, these improvements do not exceed the values predicted by the rule of mixtures, and come at the cost of decreases in other mechanical properties (ie; improved tensile strength comes at the cost of decreased impact strength).
Composites
The application of polymer biocomposite materials is becoming increasingly common in automotive components, including components such as interior panels, dashboards, liners and trims. Automotive biocomposites are generally composed of a polymer matrix, such as polypropylene (PP) or polyethylene (PE), which is blended with a natural fiber such as flax or hemp to provide reinforcement. This is analogous to synthetic materials such as fiberglass composites, in which a polymer is reinforced with glass fiber to significantly improve its strength. The benefit of utilizing natural fibers, rather than synthetic reinforcements, is that they are low cost, low density, and can be sustainably manufactured with a lower carbon footprint. This results in weight savings at a competitive price, which is crucial in an automobile industry that is striving to improve fuel efficiency in any way possible.
Polyamide is a widely known and utilized engineering polymer especially in the automotive industry. From the stand point of having excellent mechanical performance, good thermal properties and wear resistance, it is superior to many of the petroleum based polymers such as PP and PE. Polyamide has numerous commercial applications such as packaging, fiber materials, auto part application, etc. because of its large scale availability in the market at a reasonable price and ease of production. Applications of this polymer in areas that require very high strengths, moduli and sudden impact are however hindered by its relatively lower strengths and moduli in comparison to metals and inherent sensitivity to notch.
Composite materials have been applied in under-the-hood automobile components to achieve weight savings by replacing metal components with lighter polymer-based materials. Due to the high temperatures, stress and pressure experienced by under-the-hood components, these composites must utilize engineering plastics, mainly polyamides such as nylon 6 and nylon 6,6. These polymers are able to withstand high temperatures and pressures, but have high melting points, and thus require processing at high temperatures (up to 270° C.) to produce molded components. This requirement makes natural fiber reinforcements inherently inappropriate, as such biofibers burn at that high temperature. Approaches have been developed to improve the strengths and moduli of polyamide by the use of light weight reinforcing fillers such as natural fibers and fillers. However, major issues such as degradation and emission of odor from these fillers during composite fabrication have been noticed; resulting in reduction of the fiber integrity. Ozen and co-workers studied the fabrication of nylon hybrid biocomposites with a combination of different natural fibers through melt mixing at varying fiber loading between 5-20 wt % (Ozen, Kiziltas, Kiziltas, & Gardner, 2013). It was found that addition of natural fibers improved the mechanical properties significantly. However, it was observed from the morphology of the composites that the interface between the nylon and fibers was poor. The issues of thermal stability of the fibers always remain as long as there is no pretreatment done to them.
U.S. Pat. No. 7,582,241 to Mohanty, Tummala, Misra, and Drzal, describes a way to solve the issues stated above by reducing the melting temperature of nylon with the use of inorganic salts (U.S. Pat. No. 7,582,241, 2009). The resulting composites exhibited enhancement in the mechanical properties as a result. The ability to successfully incorporate natural fibers into nylon was achieved. A drawback to this process was that it required a two stage process. The first stage was the incorporation of the salt in the nylon to reduce the melting temperature, followed by the second stage of adding the fibers there after. This creates added cost to the manufacturing process and eventually to the finished composite material. To combat the low thermal stability of natural fibers, Vold et al. investigated the thermal pretreatment of sunflower hull by torrefaction and use thereof as reinforcement in nylon (Vold, Ulven, & Chisholm, 2014). They also incorporated untorrefied sunflower hull in nylon and compared the mechanical properties. It was observed that a strong odor was given off during composite fabrication with the untorrefied natural fibers, whereas the odor was not observed with the torrefied fibers as reported. It was also noticed that the torrefied natural fiber composites exhibited better tensile strength in comparison to that of the untorrefied fiber composite. However, the authors also observed some voids at the interface between the torrefied fibers and nylon matrix. It was suggested that the torrefaction process was incomplete and therefore resulted in some level of fiber decomposition.
Cellulose fibers have high thermal stability and are capable of being used as reinforcement in nylons or engineering polymers. Their nano-sized structure improves surface area contact with the matrix and allows for better load transfer. However, the difficulty in dispersing the cellulose fibers within the matrix results in agglomerates of cellulose fibers and thereby hindering the optimized properties of the composite. A study on cellulose fiber reinforced nylon composites revealed that the introduction of cellulose into nylon enhanced the tensile and flexural properties up to 30 wt % (Xu, 2008). However, the difficulty of dispersing the cellulose fibers evenly within the matrix was a problem. Longer processing time and higher shear rates will be required in improving dispersion but is hindered by the possible degradation of the cellulose fibers for long periods of processing. Tajvidi et al. studied the effect of different temperatures on the mechanical properties of cellulose reinforced nylon (Tajvidi, Feizmand, Falk, & Felton, 2008). It was found that there was a drastic decrease in properties at higher temperatures, especially that of the modulus. However, it was observed that the composite possessed better temperature resistance than pure nylon. Light weight and thermally stable carbonized lignin was used to reinforce polytrimethylene terephthalate by Myllytie et al. (Myllytie, Misra, & Mohanty, 2016). At optimized conditions, the flexural strength and modulus was observed to increase while also having improvements in the heat deflection temperature and dimensional stability. In comparison to commercially available mineral filled composite systems, the lignin-reinforced PET was found to be superior. Huber et al. studied the rheological effects of biocarbon reinforced nylon 6 (Huber, Misra, & Mohanty, 2015). They found that the reduction in particle size drastically reduced the viscosity of the composite melt and suggested that this could have an effect on the mechanical performance. This suggestion indicates that by reducing the particle size, increased surface area is achieved and better wetting of the filler by the matrix as well. In another study, biocarbon from miscanthus fibers was incorporated into nylon 6 without pretreatment such as milling or grinding (Mohanty, Vivekanandhan, Anstey, & Misra, 2015). The mechanical performance of the nylon was observed to decrease with the incorporation of biocarbon. This was due to the intrinsic defective structure which did not undergo any form of pretreatment.
From the above it can be concluded that the use of natural fiber even after torrefaction still poses significant hurdles to be utilized in high temperature composite fabrications such as nylon composites. As such, commercial under-the-hood composites are generally reinforced with glass fibers and/or mineral fillers, such as talc or clay. While these fillers are thermally stable and provide excellent improvements in mechanical properties, they are non-renewable materials with relatively high densities, which limits weight savings.